The present invention relates to a method and system for detecting interference.
Interference is a problem in many wireless signalling systems. Examples include Global Navigation Satellite Systems (GNSS). GNSS use satellites for geo-spatial positioning with global coverage. The best known example of these systems is the Global Positioning System, or GPS. Using these systems small electronic receivers are able to determine their location (longitude, latitude, and altitude) to within a few meters using the travel times of signals transmitted along a line-of-sight by radio from multiple satellites.
GPS signals are formed of a navigation message binary phase shift modulated (BPSK) onto a direct sequence spread spectrum (DSSS) signal. The spread spectrum signal comprises a unique pseudo-noise (PN) code that identifies the satellite. For civil application GPS signals transmitted using the L1 carrier frequency, this code is known as the C/A code. The C/A code is a member of a Gold code family. Other code families are in use for other global navigation satellite systems. The C/A code has a sequence length of 1023 chips and it is spread with a 1.023 MHz chipping rate. The code sequence therefore repeats every millisecond. The code sequence has an identified start instant when the two code generators in the satellite just transition to the all state. This instant is known as the code epoch. After various transport delays in the satellite, the code epoch is broadcast through the timing and sequence of specific code states assigned to the satellite. This signalling event can be recognised, in suitably adapted receivers, through a process of aligning a replica code with the code received from each satellite.
The navigation message has a data rate of 50 bits per second, lower than the code rate, and its data bit or symbol transitions are synchronised with the start of the C/A code sequence. Each bit of the navigation message lasts for 20 milliseconds and thus incorporates 20 repetitions of the CIA code. The navigation message is constructed from a 1500-bit frame consisting of five 300-bit sub-frames. Each sub-frame lasts for 6 seconds. The satellite transmits the navigation message and C/A code using a carrier frequency that is an integer multiple of 10.23 MHz (for the L1 carrier, the multiple is 154).
As mentioned above, a GPS receiver may determine the time-of-arrival of a signalling event through a process of aligning a replica code with the code received from each satellite. The receiver may also use TOW (time of week) information contained in the navigation message to determine the time when the signalling event was transmitted. From this, the receiver can determine the transit time for the signalling event (from which it can determine the distance between it and the satellite), together with the position of the satellite at the time when the signalling event was transmitted (using ephemeris information sent with the navigation signal). The receiver can then calculate its own position. Theoretically, the position of the GPS receiver can be determined using signals from three satellites, providing the receiver has a precise time reference or knowledge of part of the positions, such as altitude. However, in practice GPS receivers use signals from four or more satellites to determine an accurate three-dimensional location solution because an offset between the receiver clock and GPS time introduces an additional unknown into the calculation.
The first stage of processing during which the visible signals are identified is known as signal acquisition. To find the signals from any satellites that may be in view the receiver may search over a range of possible time and frequency offsets. Although GPS is a spread spectrum system and therefore all the satellites broadcast the C/A code on nominally the same frequency, a frequency search is still required if there is any uncertainty in the frequency of the receiver's local clock or if there is any uncertainty in the receiver's knowledge of the relative motion of the receiver and the satellites. In order to find one particular satellite with a PN known to the receiver, the receiver must compare the signal it receives to the expected signal, usually using a cross-correlation method. If the receivers time uncertainty exceeds the code duration then the code transmission can be at any point in its cycle, and the receiver must search over all possible code phases. In this specification a search for a particular PN at a particular frequency over all potential code phases is referred to as a search spanning a particular search bin.
These signal searches can mistakenly identify spurious signals caused by coherent interferers. These may be due to cross-correlation from other satellites of the same constellation, augmentation satellites, other constellations of satellites, narrowband interferers or local jammers, amongst others. Such spurious signals can result in process resources at the receiver being overwhelmed and invalid signals being identified. Interferers are commonly PN- and frequency-specific but tend to impact many code phases within a search bin.
In a system of the type described above, coherent interferers have waveforms that include repetitive characteristics, the periodicity related to the CA code repeat period. In the presence of a coherent interferer the matched filter results for a particular code phase may have a bias that can persist for a period of time. This period of time depends on the rate at which the interfering waveform drifts with respect to the matched filter code phases. The coherent interferer-induced bias creates a problem for GPS receivers because it accumulates in exactly the same way as a real GPS signal. Thus even a small coherent interferer may look to the detection logic like a real signal or it may obscure the real signal.
The conventional approach to eliminating coherent interferers is to detect strong satellites first and measure their signal strengths and frequencies. A look-up table is then used to determine if a weaker candidate satellite signal could be a further cross-correlation from one of the strong satellites. More advanced systems also consider the possibility that the weak signal is due to interference from a combination of strong satellites. Additionally the interference depends on other factors that may or may not be known such as the coarse alignment of the receiver's local clock. These factors may be individually taken into account, or a correction factor corresponding to the overall worst case scenario could be applied.
An alternative technique is to determine the correct code phase alignment for each of the satellites from which strong signals are being received and the magnitude of the received signals. The cross-correlation values of these strong satellite signals with all the weaker ones still subject to search can then be determined. A process of subtraction may then be employed to remove the effects of the cross-correlation from all the strong satellite signals from the channels used to search for the weaker satellite signals. This process can significantly improve the detection sensitivity for the weaker satellite signals. However, it should be noted that this technique only works well when the relative distances to each satellite are known to within ½ chip, otherwise the cross-correlation subtraction does not take place in the correct respective search bin.
These existing methods require detailed knowledge of parameters relating to the potential interferers including their identity, signal structure and signal strength. This parametric information is then used to work out the expected interference and discard signals from search bins which are identified as potentially compromised. Therefore all possible interferers should be known and continuously monitored. This approach does not scale well in the presence of more than one interferer and therefore, as more GNSS systems are deployed, this is an increasingly demanding task. The emerging multiplicity of GNSS signal sources, especially in the L1 band, will undoubtedly increase the global interference suffered by GNSS receivers at least due to the cross-correlation effects between the various code families used. There is clearly practical difficulty in obtaining and maintaining a list of all potential interferers. Additionally, complications arise when multiple coherent interferers are present. A general mechanism for coherent interference rejection would be preferred over this parametric mechanism so that a receiver could be resilient to any present or future coherent interferer. What is needed is a signal acquisition method, implementable on relatively simple hardware, which can eliminate coherent interference signals from signal acquisition without detailed parametric knowledge of interferers.